This invention relates to electrostatic discharge (ESD) generators/simulators and particularly to devices which simulate positive and/or negative charged ESD that can occur when a human or an object, directly or indirectly, touches electronic equipment. This phenomenon is commonly known as static electricity.
ESD is generated when two objects with different electrical potential approach near each other such that an electric charge is transferred from one object to the other. Direct contact between the two objects is not required, as the static charge will jump or arc through the air (air gap) when the two objects are in close proximately. Inanimate objects may also generate ESD when moved in close proximately to each other, and this is commonly referred to as furniture ESD. Human ESD is generated when ESD jumps from a person to an object or other person. Naturally occurring ESDs have a very inconsistent electrical characteristic, dependent upon environmental conditions, object surfaces, and electrical potential differences.
In many instances, human ESD or furniture ESD can cause electronic equipment to malfunction. Equipment failures may also result from non-direct ESD, in that the equipment is not one of the two objects involved in the ESD generation. With direct ESD, the equipment will be subjected to voltage and current transients. All ESD will also generate non-direct electromagnetic interferences: an electric field (E-field) and a magnetic field (H-field). Individually or combined, all electromagnetic transients produced by direct and/or non-direct ESD can cause electronic equipment to malfunction. Therefore, it is important for manufacturers to be able to test the vulnerability of their electronic equipment to ESD. Further, it is expected that in the fixture it will be required that electronic equipment be tested to ensure that it meets certain minimum standards. As discussed below the International Electrotechnical Commission is developing standards for ESD testing. It is expected that eventually these standards will be adopted by member countries and equipment will be required to meet these standards.
There are several arrangements known in the prior art for simulating ESD. ESDs are not easily controlled because of their extremely fast rise time (in picoseconds) and their short pulse life (in nanoseconds) which are a result of air gap arcing between the two objects. Known ESD simulators are typically expensive, large and require a capacitor and an external alternating current or battery source.
The International Electrotechnical Commission (IEC) is developing a standard for ESD testing of digital electronic equipment. In 1988 a draft standards was produced entitled "Draft-Revision of IEC Publication 801-2 Electromagnetic compatibility for industrial process measurement and control equipment--Part 2: Electrostatic discharge requirements", (referred to as "IEC Standard 801-2"). This standard identifies two methods for testing human ESD: direct contact discharge and air discharge. In direct contact, the recommended simulator is charged to a predetermined voltage potential using an electrical capacitor (150 pF); a contact point is made to the equipment-under-test (EUT) and the electric discharge is generated by shorting the capacitor by means of relay contacts. The short causes the capacitor to discharge whereby the current is discharged through a resistor (330 Ohms) and then through the contact point into the EUT. A ground wire between the simulator and the EUT completes the circuit loop.
A direct contact discharge simulation is consistent but cannot give a reasonable approximation of a human discharge because it lacks the electrical arcing characteristics associated with human discharges. Under an air discharge test, the same simulator is used except the means of discharge is different. Specifically, the simulator is first charged then brought close to the EUT causing an electrical arc discharge to occur in the air gap between the EUT and the simulator. The discharge in this method is uncontrolled because the electrical characteristic of the arc is a function of the environmental conditions, the air gap distance, and the rate at which the simulator is moving toward the EUT.
Current simulators on the market cannot reproducibly control the characteristic of the ESD in the air discharge, hence a great number of tests must be performed to obtain a reasonable test result. Thus, most ESD testing uses the unrealistic non-arcing direct contact method. Further, simulators currently in use include a ground wire to complete the electrical circuit, however, the use of this ground wire introduces an impedance which can have adverse effects on the test.
It is an object of the present invention to provide a hand held trait with an internal source of energy, not requiring a ground wire, that generates an ESD with a readily controllable, reproducible electromagnetic discharge and which can be applied to the EUT directly. In effect, the present invention is designed to be easily used and to simulate more closely human ESD. In addition, the present invention may be adapted to use alternative probes to generate E-fields and H-fields, the other two components of the electromagnetic interference that an electrostatic discharge can produce.